The present invention discloses a method for preparing a cationic cross-bonding reagent and a process for preparing cationic cross-bonded starches with tailor-made viscosity, and the use of said cationic cross-bonded starches in the wet-end of papermaking. More particularly, the present invention relates to the preparation of cationic cross-bonding reagent wherein the amount of cross-bonding impurities is determined and a certain defined quantity of a cross-bonding agent is added. Applying this cationic cross-bonding reagent results in cationic cross-bonded starches with tailor-made, constant viscosity and excellent viscosity stability. Products with tailor-made viscosity can be used according to their need in specific applications in the wet-end of papermaking.
Cationic starches are well known in starch technology and have long been used principally as wet-end additives in the manufacture of paper to increase wet strength and pigment retention. In addition it has been found that in the recycle of starch-sized paper it is important to have as little as possible losses of cationic starch to the white water during re-pulping operation, consequently resulting in lower BOD and COD values of the white water.
Nowadays the cationisation is most frequently performed in presence of commercially available cationic agent, such as 3-chloro-2-hydroxypropyl trialkylammonium chloride, preferably 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHPT).
U.S. Pat. No. 5,122,231 describes the cationisation of granular starch in the presence of commercial cationic agent such as 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, followed by the cross-linking of the obtained cationic starch with any kind of cross-bonding agent. The viscosity of the thus prepared cationic cross-bonded starches is within a very broad range of from about 500 to about 3000 mPa.s (Brookfield at 1.4% starch at 95xc2x0 C., after a period of 10 minutes, at 20 rpm (No. 21 spindle).
This process does not allow preparing cationic cross-bonded starches with tailor-made, constant viscosities within a specific and narrow range. When applying repeatedly identical reaction conditions by adding identical quantities of commercial grade of cationic agent and identical amounts of a cross-bonding agent, starches with a broad range of fluctuating viscosities are obtained and off-code products are the result. These fluctuations are related to the purity of the cationic agent.
The preparation and partial purification of this cationic agent has been investigated intensively.
U.S. Pat. No. 4,602,110 describes a method to purify 3-chloro-2-hydroxypropyl trialkylammonium chloride from 1,3-bis(trialkylammonium chloride)-2-hydroxypropane (di-quaternary product or Diquat) and 1,2-hydroxypropane-3-trialkylammonium chloride. These products are non-reactive to starch manufacturing process, and are typical by-products formed during the reaction of epichlorohydrin with trialkylammonium hydrochloride.
U.S. Pat. No. 5,077,435 describes a method to reduce the formation of especially those non-reactive di-quaternary by-products by the use of 1,3-dihalo-2-propanol as co-solvent.
U.S. Pat. No. 5,463,127 recognises that besides those non-reactive by-products such as 1,3-bis(trialkylammonium chloride)-2-hydroxypropane, reactive cross-bonding by-products might be present in the cationic agent. These reactive by-products, such as residual epichlorohydrin and 1,3-dihalo-2-propanol are preferably removed by solvent extraction or vacuum distillation prior to the use of the cationic agent for cationisation of starch.
Although the presence of these reactive and non-reactive impurities in the cationic agent is recognised and several purification steps are performed to meet the purity specifications of the commercial grade of the cationic agent, cationisation in presence of this cationic agent still gives cationic starches with fluctuating viscosities, such as is described before.
Accordingly, a need exists for a cationic cross-bonding reagent of constant quality and a process, wherein by applying prescribed reaction conditions with defined quantities of cationic cross-bonding reagent, cationic cross-bonded starches with tailor-made viscosities (Brookfield) are prepared. In repeated trials with identical reaction conditions, products with similar Brookfield viscosity should be obtained.
The current invention discloses a method for preparing a cationic cross-bonding reagent comprises reacting a trialkyl amine or a trialkylammonium chloride with epichlorohydrin for obtaining cationic agent and is further characterised in that:
a) The amount of bis(2,3-epooy propyl)alkylamine and/or bis(2,3-epoxy propyl) dialkyl ammonium chloride in the cationic agent is determined, and
b) A defined quantity of a cross-bonding agent between 0 to 15 mmoles per kg dry cationic agent is added.
The current invention further relates to a method wherein the cationic agent is (3-chloro-2-hydroxypropyl)trialkyl ammonium chloride, or N-(2,3-epoxypropyl)trialkyl ammonium chloride and alkyl can be from C1 to C8, preferably (3-chloro-2-hydroxypropyl)trimethyl ammonium chloride, or N-(2,3-epoxypropyl)trimethyl ammonium chloride.
The cross-bonding agent is selected from the group consisting of polyepoxide compounds, such as polyamine polyepoxide resin, or phosphorous oxychloride, cyclic sodium trimetaphosphate, 1,4-alkanediol diglycidyl ether, dihalo-propanol, dimethylol-ethylene urea, or bis-reagents such as bis(2,3-epoxy propyl)alkyl amine and/or bis(2,3-epoxy propyl)dialkyl ammonium chloride wherein alkyl can be from C1 to C8.
The current invention relates to a method wherein the amount of bis(2,3-epoxy propyl)alkylamine and/or bis(2,3-epoxy propyl)dialkyl ammonium chloride is determined by chromatography, preferably by high performance liquid chromatography (HPLC) of the cationic agent, or by measuring the Brookfield viscosity of the cationic starch prepared with cationic agent.
The present invention further relates to a process for preparing cationic cross-bonded tarches of tailor-made, predetermined and constant Brookfield viscosity between 600 to 2000 mPa.s (measured as Brookfield viscosity of 3% paste at 50xc2x0 C. and pH 6), preferably between 800 to 1700 mPa.s, wherein said process is comprising a cationisation step and a cross-bonding step with a cationic cross-bonding reagent and wherein the cross-bonding is performed simultaneously and/or subsequently to the cationisation step, and said process is characterised in that the cationic cross-bonding reagent is prepared according to the aforementioned method.
The current invention relates to a method for papermaking comprises the addition of the aforementioned cationic cross-bonded starch to the pulp at the wet-end step of the papermaking process and in an amount of between 0.3 and 3 weight percent of the pulp, preferably between 0.6 and 1.5 weight percent of the pulp.